SOLAR ACTIVITY

The surface of the Sun is a spectacular, turbulent place, and signs of intense solar activity, such as solar flares and sunspots, can be seen from Earth.

The visible surface of the Sun is called the photosphere ("sphere of light"). It looks solid, but in reality it is a shell of gas about 250 miles (400 km) thick. At about 11,000 degrees Fahrenheit (6,000° C), this outer layer is much cooler than the gas deep beneath it.

The edge of the photosphere looks dark compared to the center of the disk. This limb-darkening effect occurs because at the center of the disk we see deeper into the Sun where temperatures are higher and the gas shines more brightly. At the limb we are seeing cooler gas at higher elevations and it appears darker.

Viewed at high magnification, the photosphere shows many features. The smallest of these are individual cells of gas called granules, which average about 700 miles (1,100 km) across. Granules can last from several minutes to half an hour, but are usually difficult to observe because of the poor seeing during daytime.

The photosphere also has large, irregular patches of slightly hotter material called faculae (Latin for "torches"). Because they are not much brighter than the rest of the surface, they are often hard to see, except near the limb.

With a standard H-alpha filter, you may see bright loops called prominences extending from the Sun's disk. They can also be seen with a narrowband H-alpha filter as dark filaments on the disk itself.
Another kind of activity—a flare—is always worth looking for, although it is rare to see one. The first was discovered in 1859 by two English amateurs, Richard Carrington and Richard Hodgson, who were working independently. Flares are brilliant explosions of pent-up magnetic energy and radiation. They come and go, often within minutes, but most are invisible without an H-alpha filter. White-light flares, which can be seen with any solar filter, are rare but powerful events. They may release as much as 2 percent of the Sun's energy from a tiny area in a few minutes, and can disturb the electromagnetic environment of the whole inner Solar System. When the flare's particles arrive at Earth as a high-speed gust in the solar wind, they create geomagnetic disturbances such as auroras. In extreme cases, they can produce surges of current that cause power grids to tail, leading to widespread blackouts.

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